A liquid crystal display operates at low voltage as well as low power consumption, and further, can be directly connected to an IC circuit. Specifically, it is possible to decrease its thickness. As a result, it is widely employed in word processors, personal computers, and the like, as the display. The basic structure of said liquid crystal display is such that polarizing plates are provided on the both sides of the liquid crystal cell.
A liquid crystal display has characteristics in that it operates at low voltage as well as at low power consumption, and decreases its thickness, which other displays do not have. Problem of the liquid crystal display is one in which a viewing angle is small. Solution of the problem has been eagerly required, and technique to solve this problem has been developed. As the technique, there has been proposed an optical compensation film comprising an optically anisotropic layer containing a discotic liquid crystal compound obliquely oriented, which is provided on a polarizing plate of a TN (TN-TFT) mode liquid crystal display. However, further wide viewing angle in, for example, has been increasingly demanded year by year.
As one of the means solving the above problems, there has been proposed a liquid crystal of type different from a TN or STN type liquid crystal. A vertical alignment (VA) mode liquid crystal, for example, a negative liquid crystal having a negative dielectric anisotropy, has been developed, in which when no voltage is applied, liquid crystal molecules are oriented vertical to the orientation plate, while when voltage is applied, they are oriented parallel to the orientation plate. In the TN or STN type liquid crystal, when no voltage is applied, liquid crystal molecules are oriented parallel to the orientation plate, while when voltage is applied, they are oriented vertical to the orientation plate. A VA mode liquid crystal display comprises a liquid crystal cell with a vertical alignment mode in which when no voltage is applied, liquid crystal molecules are oriented vertical to the orientation plate, while when voltage is applied, they are oriented parallel to the orientation plate.
This VA mode liquid crystal display comprises a liquid crystal cell with a vertical alignment mode in which when no voltage is applied, liquid crystal molecules are oriented vertical to the orientation plate, while when voltage is applied, they are oriented parallel to the orientation plate. As a result, in such a liquid crystal display, black is displayed as genuine black, and contrast increases. Particularly in a VA mode liquid crystal display multi-divided, it has been possible to obtain a viewing angle of 160° at the upper, lower, left and right portions of the display. However, as the size of a liquid crystal display increases, further increase of the viewing angle, particularly the viewing angle in the oblique direction of 45° (in the direction of 45°, 135°, 225°, 315°) has been increasingly demanded.
In order to increase the viewing angle of the VA mode liquid crystal display, the present inventors has made an extensive study on a polarizing plate protective film. During the course of the investigation, and as a result, they have found that when a film is employed in which an retardation value (Rt value) in the thickness direction, showing anisotropy in the in-plane direction as well as in the thickness direction, is adjusted to a positive value, that is, a so-called C-plate in which anisotropy in plane is adjusted to a minimum, and a retardation in the thickness direction to a specific range, is used, the viewing angle of the VA mode liquid crystal display further increases (Japanese Patent O.P.I. Publication No. 2001-188128). However, this method could not prevent the thickness from increasing, although it is smaller in the thickness as compared with those in which a predetermined Rt is obtained by laminating plural conventional polarizing plate protective films.
It is known that insertion of a phase difference plate with optical biaxiality of a resin such as polycarbonate between a polarizing plate and a liquid crystal cell increases the viewing angle described above, the optical biaxiality being provided by biaxially stretching the phase difference plate, and for example, “VAC film” produced by Sumitomo Kagaku Co., Ltd. is available on the market. However, a phase difference plate of a resin such as polycarbonate has problems in uniformity of phase difference or in transmittance, and is required to be laminated onto a polarizing plate every piece, which results in problems in productivity or cost of manufacture.
An optical film, particularly a polarizing plate protective film is required to have resistance to heat or humidity, which prevents its shrinkage or deterioration due to moisture absorption of a polarizer at high temperature and high humidity, and separation the polarizing plate from a glass plate of the liquid crystal cell due to deterioration of the adhesive used. Further, the optical film is required to have high transparency, high strength, or ease of handling.
Further, when cellulose ester film is produced by dissolving cellulose ester and casting the resulting solution, cellulose ester in which the esterification has been insufficient tends to remain in the film as insoluble foreign materials. When such a film is incorporated to a liquid crystal display element, the insoluble foreign materials causes defects that disturb the polarizing state to emit abnormal light due to its difference in the refractive index from cellulose ester film, so-called defects due to luminescent foreign materials. These defects have been less problematic in a TN mode (TN-TFT) liquid crystal display, which is an optical rotatory mode, but it has been found that they are problems to be solved in a VA mode liquid crystal display, which is a birefringence mode.
Japanese Patent O.P.I. Publication No. 9-90101 proposes that casting can be carried out employing a solvent other than chlorine-containing hydrocarbons as the solvent, where incorporating a specific amount of an acetyl group or a propionyl group to cellulose to obtain a cellulose ester, selection range of the solvent of the cellulose ester increases, and also proposes that fatty acid cellulose esters having a low retardation value in both in-plane and thickness directions are used for the purpose such that high contrast of a TFT mode or FSTN mode liquid crystal display realizing high contrast is not jeopardized.
However, when used in the VA mode liquid crystal display, any proposals described above are not satisfactory in view of resistance to heat and humidity, the number of foreign materials occurring or increase in viewing angle. Further improvement is required.